Occasionally, in the drilling of oil and gas wells and particularly in the subsea drilling of oil and gas wells, it is necessary, as an emergency procedure, to shear a drill string and shut off a well. This is accomplished generally by the use of blowout preventers provided with shear rams. Typical of such blowout preventers provided with shear rams are those disclosed in the following U.S. Pat. Nos. 2,919,111, granted Dec. 29, 1959, to K. Murray Nicolson; 2,969,838, granted Jan. 31, 1961, to Arthur E. Wilde; 3,561,526, granted Feb. 9, 1971, to Williams, et al; 3,590,920, granted July 6, 1971, to Orund, et al.; 3,736,982, granted June 5, 1973, to Ado Vujasinovic; and 3,817,326, granted June 18, 1974, and 3,946,806, granted Mar. 30, 1976, to Maurice J. Meynier.
In drilling many wells, an hydrogen sulfide environment is encountered which causes hydrogen embrittlement and hence failure of shear rams having the strength and hardness levels sufficient to shear the heavy drill strings as in a non-hydrogen embrittlement environment. For example, when hydrogen sulfide contacts shear rams made from a steel alloy having a strength and hardness level exceeding that of the drill string to be sheared, say Rc35 - 40, the hydrogen sulfide decomposes to form a metal sulfide and to liberate atomic hydrogen which diffuses into the metal lattice. The metal is then said to be hydrogen embrittled. If a metal is subjected to high stresses, such as in shearing drill strings or in containing high well pressures, brittle failure occurs at stress levels many times lower than the stress required for failure in the absence of hydrogen.
It has been found that alloy steels not exceeding a hardness level of Rc22 may be used successfully in hydrogen sulfide environments since these alloys are tough, resist crack propagation and do not become hydrogen embrittled under stresses. Alloys of such a hardness level, however, have a strength and hardness level less than that of the drill string, and, accordingly, upon engagement with the drill string, these alloys would be deformed and would not shear the drill string, since it is necessary that the hardness of the shear rams exceed the hardness of the drill string in order to prevent deformation and to provide shearing of the drill string.
Shear rams made of alloys of Rc35 - 45 have a strength and hardness which exceed the hardness of most drill strings and thus are not deformed by the drill string and shear the drill string upon engagement with it; however, such alloys are subject to hydrogen embrittlement when used in a hydrogen sulfide environment, and thus crack and fail to shear heavy drill strings. To the inventor's knowledge, prior to the present invention no shear rams were marketed or available which would shear successfully in an hydrogen sulfide environment.
One solution to the problem is to make the shear rams from exotic metals not subject to hydrogen embrittlement which do have a sufficient level of hardness to bite into and shear the drill string without deformation; for example, A-286, MP35N, Waspalloy, Inconel, and Rene' 41. However, these metals are extremely expensive and, hence, it is impractical to make shear rams of these metals.
The foregoing problems and disadvantages are solved and overcome by the provision of, and the present invention is directed to, shear rams having bodies formed of a relatively soft ductile metal, that is, having an upper hardness level of about Rc22, provided with pipe-shearing and pipe-engaging faces or surfaces formed of a relatively thin layer of nickel-based alloys, cobalt-based alloys, and tunsten-based alloys. Unexpectedly, these shear rams are strong enough to shear drill strings passing through blowout preventers and successfully shear drill strings in hydrogen sulfide environments.
The inventor is not aware of any prior publications or uses of shear rams formed of metal alloys having an upper hardness level of Rc22 and provided with shearing faces and drill string pipe-engaging faces formed of nickel-based alloys, cobalt-based alloys, or tungsten-based alloys. The inventor is aware of U.S. Pat. No. 3,880,436, granted Apr. 29, 1975, to Jose Robert Canal, which discloses ram blocks having bodies formed of an alloy having an upper hardness level of Rc22 provided with a supporting inner portion of a relatively soft, work-hardenable alloys, which when relatively stress-free have an upper hardness level of about Rc22 and are not subject to failure because of hydrogen embrittlement, which, upon energizing the ram blocks to bring them into engagement with the tool joint, rapidly workharden sufficiently to indent themselves into the tool joint and support the drill string load without substantial deformation. This solution is not satisfactory, however, since the inlay is initially softer than the tool joint, it tends to deform and loses its sharp edge and hence does not shear the drill string. In addition, these work-hardenable inlays do not work-harden sufficiently and they crack, particularly at corners, and hence leak.